A prior an scanning device 108, as shown in FIG. 1, is typically used to read symbology 110, such as a bar code, which consists of alternating areas ("bars") having different reflectance/absorption characteristics. The scanning device 108 receives the reflected light and interprets the fluctuations in radiant emittance caused by the reflectivity characteristics of the symbology 110. When the scanning device 108 is held in a stationary position, the object beating the bar code symbology 110 must be brought within the working range of the scanning device 108. This can be done with a conveyor belt 123. In "over-the-belt" scanning applications where objects 150 of different sizes travel underneath the scanning device 108, the distance from the scanning device 108 to an object 114 being scanned will change based on the size (height) of the object 114. When the optical path length 120 between the scanned object 114 and the scanning device 108 changes significantly, a number of problems will be encountered. The most obvious problem is loss of focus. This can be corrected with an automatic focusing mechanism built into a scanning lens system 112. The less obvious problem is that of changes in magnification. To compensate for such changes one must use a variable focus (zoom) system. The most practical and effective zoom system is based on changing the focal length of the scanning lens system 112. To achieve changes in focal length one or more lens groups must be moved within the scanning lens system 112.
Typically, zoom lens systems for industrial applications will be custom made when image quality and long life span are critical. This means that such variable focal length lenses are more expensive. Due to the need for the scanning lens system 112 to move lens group(s) for zooming and focusing, the optical performance of the system will be lower. Such zoom lenses typically have a lower modulation transfer function (MTF) than a fixed focal length lens, where MTF describes the modulation (contrast) of the image as a function of the spatial frequency of the object. Moving lenses will also place strict requirements on the actuators, which must move lenses with high precision very fast through millions of cycles. Any tilt or sideways shift introduced through the motion of the moved lenses will further degrade the image quality. Accordingly, "off-the-shelf," inexpensive lenses cannot be used in the scanning lens system 112 when the optical path length 120 significantly varies outside of the depth of field of the scanning lens system 112 while reading a sequence of symbologies 110 on different size objects 150. In the past, one was restricted to either using multiple scanners or lower performance but expensive custom lens systems.